Solderless die attach to a direct bonded aluminum substrate

ABSTRACT

A DBA-based power device includes a DBA (Direct Bonded Aluminum) substrate. An amount of silver nanoparticle paste of a desired shape and size is deposited (for example by micro-jet deposition) onto a metal plate of the DBA. The paste is then sintered, thereby forming a sintered silver feature that is in electrical contact with an aluminum plate of the DBA. The DBA is bonded (for example, is ultrasonically welded) to a lead of a leadframe. Silver is deposited onto the wafer back side and the wafer is singulated into dice. In a solderless silver-to-silver die attach process, the silvered back side of a die is pressed down onto the sintered silver feature on the top side of the DBA. At an appropriate temperature and pressure, the silver of the die fuses to the sintered silver of the DBA. After wirebonding, encapsulation and lead trimming, the DBA-based power device is completed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority under 35U.S.C. §120 from, nonprovisional U.S. patent application Ser. No.13/490,459 entitled “Solderless Die Attach to a Direct Bonded AluminumSubstrate,” filed on Jun. 7, 2012, now U.S. Pat. No. 8,716,864, thesubject matter of which is incorporated herein by reference.

TECHNICAL FIELD

The described embodiments relate to DBA-based power devices.

BACKGROUND INFORMATION

Direct-Bonded Copper (DBC) substrates (also called Direct Copper Bonded(DCB) substrates) are used extensively in the power semiconductorindustry. A DBC substrate generally includes a thick ceramic substratemember, to which a thinner top plate of copper and a thinner bottomplate of copper are bonded. A semiconductor die includes one or morepower devices such as, for example, power transistors and/or powerdiodes. Metallization on the back side of this semiconductor die issoldered to the top copper plate of the DBC. The DBC is often physicallysecured by soldering or welding the top copper plate to a package lead.Bond pads on the front side surface of the die are typically wirebondedto other package leads. The resulting assembly is then encapsulated.After lead trimming, ends of the leads protrude from the encapsulant. Toimprove conduction of heat from the package, the encapsulant is made notto cover the bottom surface of the bottom copper plate. This exposedsurface of the DBC facilitates heat escaping from the package. Theexposed surface may, for example, be placed in good thermal contact withan external heat sink. For additional background information on DBCsubstrates and packaged power devices that include DBC substrates, seeU.S. Pat. No. 6,404,065.

Although DBC-based power devices work well in many applications, otherapplications involve thermal cycling over wider temperature ranges andto higher temperatures. In such applications, differential thermalexpansion between the DBC ceramic and copper may result in cracking ofthe ceramic or in separation at the ceramic-to-copper interface.Accordingly, a metal bonded ceramic substrate referred to as a DBA(Direct-Bonded Aluminum) substrate has been developed. DBA substratesare sometimes called Direct Aluminum Bonded (DAB) substrates. Ratherthan direct bonding copper plates onto ceramic as in the case of a DBCsubstrate as described above, aluminum plates are direct bonded ontoceramic. Aluminum is a softer and more ductile metal than copper.Aluminum has a lower crystal hardness modulus than copper. As a resultof being softer, the aluminum conforms with less stress to the expansionand contraction of the alumina ceramic substrate. Thus DBA substratesgenerally provide better resistance to cracking and lift off failuresunder severe thermal power cycling.

Aluminum is not, however, a readily solderable metal. The top surface ofthe top side aluminum plate of the DBA is therefore provided with a thinlayer of a solderable metal (for example, copper, or nickel, or silver,or gold). This thin layer of solderable metal allows a metalized backside of a semiconductor die to be soldered to the upper surface of thetop aluminum plate of the DBA. As in the case of DBC-based packages,bond pads on the front side of the die are wirebonded to package leads.Encapsulation and lead trimming are then carried out. For additionalbackground information on DBA substrates and on packaged power devicesthat include DBA substrates, see: U.S. Pat. No. 6,798,060 and U.S. Pat.No. 7,005,734.

In even more severe temperature cycling applications, DBA-based powersemiconductor devices have been noticed to fail. Further improvements indirect metal bonded semiconductor packages are sought.

SUMMARY

A DBA-based power device includes a DBA (Direct Bonded Aluminum)substrate. To make the DBA substrate, silver nanoparticle paste isdeposited onto a top side metal plate of a DBA panel assembly structure.The silver nanoparticle paste is sintered to form a sintered silverfeature (also called a sintered silver structure or a sintered silverlayer). The resulting DBA panel assembly is cut into individual DBAsubstrates. A DBA substrate is then physically attached (for example, isultrasonically welded) to a lead of a leadframe.

Silver is deposited onto the back side surface of a wafer (for example,by evaporative deposition). The wafer is then singulated into aplurality of semiconductor die. Each of the semiconductor dice thereforehas a silvered back side surface.

In a solderless silver-to-silver die attach process, the silvered backside surface of one of the semiconductor dice is pressed down onto acorresponding sintered silver feature on the top side of one of the DBAsubstrates. At an appropriate temperature and under an appropriatepressure, the silver layer of the semiconductor die fuses to thesintered silver layer of the DBA substrate. Bond pads on the face sideof the semiconductor die are then wirebonded to corresponding otherleads of the leadframe as appropriate. The DBA portion of the leadframeassembly is then encapsulated with injection molded plastic. The leadsare cut from the leadframe resulting in the completed DBA-based powerdevice.

In a first example, aluminum islands on the top side of the DBA panelassembly are plated with another metal or metals (for example, nickel orpalladium). Silver nanoparticle paste is deposited onto this layer ofplating metal. The nanoparticle paste is then sintered so that theresulting sintered silver features are disposed on the metal platinglayer. The metal plating layer may be a fifty micron thick layer ofnickel or palladium plating on the aluminum islands on the top side ofthe DBA panel assembly. The plating metal can be part of a multilayerplate involving an aluminum layer, so that both the aluminum and theplating layer are attached to the ceramic substrate of the DBA panelassembly at the time of direct bonding. The plating metal can also beapplied onto an already patterned aluminum layer of the DBA panelassembly. Regardless of which way is used to provide the thin metalplating layer, in the final DBA-based power device there is a metalplating layer disposed underneath each sintered silver feature betweenthe sintered silver feature and the underlying aluminum island of theDBA substrate.

In a second example, there is no other layer of metal plating on thealuminum islands on the top side of the DBA panel assembly. Rather, thesilver nanoparticle paste is deposited onto a thin native aluminum oxideon the aluminum islands, and sintering is carried out such that theresulting sintered silver features are disposed on the aluminum islandswithout there being any intervening other metal being between thesintered silver and the aluminum of the islands. In one example, thesilver nanoparticle paste includes particles of a flux material. In oneexample, the flux particles are a salt of hexafluorosilic acid. The fluxparticles decompose at elevated temperatures forming reactive agentssuch as an acid. During the sintering process, these reactive agentshelp penetrate the native aluminum oxide under the nanoparticle paste.Ultrasonic energy may be applied to assist in cracking the nativealuminum oxide. At elevated temperatures later in the sintering process,the flux residue and other organics are burned out of the sinteringmaterial. For information on the burning off of fatty aciddispersant/binder, see: U.S. Patent Application PublicationUS2012/0055707, to Schafer et al. (the subject matter of which isincorporated herein by reference). Accordingly, in the final DBA-basedpower device, there is either no native aluminum oxide or there is onlya penetrated native aluminum oxide layer disposed between a sinteredsilver feature and the underlying aluminum island of the DBA substrate.

In a third example, there is no other layer of metal plating on thealuminum islands on the top side of the DBA panel assembly. Any nativeoxide on localized areas on top surfaces of the aluminum islands ismechanically removed. In one example, the thin layer of aluminum oxideon one such localized area is mechanically removed by using amicro-nozzle to blast a stream of small abrasive particles at thelocalized area. This blasting is done in an atmosphere substantiallydevoid of oxygen and devoid of moisture. The spent abrasive particlesand any resulting debris are removed by an accompanying vacuum nozzle.The blasting mechanically removes aluminum oxide that may have grown onthe localized area. After this mechanical cleaning step, silvernanoparticle paste is micro-jetted onto the cleaned aluminum surfaceusing another micro-jet nozzle. Each localized area on an aluminumisland that is to bear a sintered silver feature is cleaned and thendeposited with silver nanoparticle paste in this way. The DBA panelassembly remains in an oxygen free environment between the cleaning stepand the depositing of paste step, so aluminum oxide cannot regrow on thecleaned aluminum surface before the depositing step takes place.Sintering is then carried out on the entire panel assembly, therebyforming the desired sintered silver features. The DBA panel assembly isthen cut to form individual DBA substrates. Accordingly, in the finalDBA-based power device, there is no native aluminum oxide disposedbetween a sintered silver feature and the underlying aluminum island ofthe DBA substrate.

Further details and embodiments and techniques are described in thedetailed description below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 is a perspective diagram of a ceramic panel.

FIG. 2 is a cross-sectional diagram of the direct bonding of aluminumplates to the ceramic panel of FIG. 1.

FIG. 3 is a perspective diagram of the ceramic panel after patterningand etching of the aluminum plates.

FIG. 4 is an illustration of a step of cleaning native aluminum oxidefrom the aluminum surfaces of the panel.

FIG. 5 is an illustration of plating a metal onto the cleaned aluminumsurfaces of the panel.

FIG. 6 is a cross-sectional diagram of the plated panel assembly.

FIG. 7 is a table that sets forth the constituents of a silvernanoparticle paste.

FIG. 8 is an illustration of the silver nanoparticle paste of FIG. 7being deposited onto islands of the panel assembly.

FIG. 9 is a simplified cross-sectional diagram of a volume of depositedsilver nanoparticle paste prior to sintering.

FIG. 10 is an illustration of the sintering of the volume of paste ofFIG. 9.

FIG. 11 is a simplified cross-sectional diagram of the sintered silverfeature that results from the sintering of the volume of paste of FIG.9.

FIG. 12 is a perspective diagram of the panel assembly after thesintering process has been completed.

FIG. 13 is a cross-sectional diagram of one of the DBAs that is cut fromthe panel of FIG. 12.

FIG. 14 is an illustration of two semiconductor dice being die attachedto sintered silver features on the top side of the DBA of FIG. 13.

FIG. 15 is a simplified perspective diagram that illustrates thewirebonded power device structure.

FIG. 16 is a circuit diagram of the power device structure.

FIG. 17 is a perspective diagram of the packaged DBA-based power device.

FIG. 18 is a flowchart of a method of making the packaged DBA-basedpower device of FIG. 17.

FIG. 19 is a flowchart of the second embodiment of a method of makingthe packaged DBA-based power device of FIG. 17.

FIG. 20 is a table that sets forth the constituents of a silvernanoparticle paste used in one example of the method of FIG. 19.

FIG. 21 is a simplified cross-sectional diagram of a volume of depositedsilver nanoparticle paste prior to sintering in one example of themethod of FIG. 19.

FIG. 22 is an illustration of the sintering of the volume of paste ofFIG. 21.

FIG. 23 is a simplified cross-sectional diagram of the sintered silverfeature that results from the sintering of the volume of paste of FIG.21.

FIG. 24 is a flowchart of another method of making a DBA-based powerdevice whereby native aluminum oxide is mechanically removed fromlocalized areas of the aluminum islands (for example, by micro-jetblasting a stream of abrasive particles onto each area to be cleaned)and thereafter silver nanoparticle paste is deposited onto each suchcleaned localized area.

FIG. 25 is a cross-sectional diagram showing one way that a localizedarea on the top of an aluminum island can be mechanically cleaned ofnative aluminum oxide in the method of FIG. 24.

FIG. 26 is a cross-sectional diagram showing one way that silvernanoparticle paste can be deposited onto the mechanically cleanedlocalized area in the method of FIG. 24.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. In the description and claims below, when a first object isreferred to as being disposed “over” or “on” a second object, it is tobe understood that the first object can be directly on the secondobject, or an intervening object may be present between the first andsecond objects. Similarly, terms such as “front”, “back”, “top” and“bottom” are used herein to describe relative orientations betweendifferent parts of the structure being described, and it is to beunderstood that the overall structure being described can actually beoriented in any way in three-dimensional space.

FIGS. 1-18 set forth a method of making a novel DBA-based power device 1as illustrated in FIG. 17. The DBA-based power device 1 includes no softsolder disposed anywhere under the block of encapsulant 2 of the device.“Soft solders” melt at a temperature lower than 450° C., and typicallycontain lead, tin, antimony or a combination of these. Rather thanemploying a soft solder for die attach, features of sintered silver areformed on a top side aluminum plate of a DBA within the package, and asilver surface on the back side of a semiconductor die is die-attacheddirectly to the sintered silver feature without any intervening softsolder. Bond pads on the top side of the die are wirebonded via aluminumbond wires to leads of the package. Accordingly there are no soft solderconnections in packaged power device 1. Thermal fatigue failures thatwould otherwise occur as a result of severe temperature cycling and hightemperatures were soft solder used for die attach are avoided due to thesolderless, silver-to-silver die attach process used.

FIG. 1 is a simplified perspective view of a panel 3 of a ceramicmaterial. The ceramic material may, for example, be alumina (Al2O3) oraluminum nitride (AlN). The ceramic panel may, for example, measureapproximately 5.6 inches by approximately 7.7 inches, and may be 0.63 mmthick.

FIG. 2 illustrates a step of direct bonding a first plate 4 (also calleda “front side plate” or a “die side plate”) of aluminum and a secondplate 5 (also called a “back side plate”) of aluminum to the ceramicpanel 3 of FIG. 1. In one example, the bottom side of the top side plate4 includes a layer of Al—Si alloy. The entire plate 4, including theAl—Si layer, may be 0.30 mm thick. Also, the top side of the bottom sideplate 5 similarly includes a layer of Al—Si. The entire plate 5,including the Al—Si layer, may be 0.30 mm thick. Al—Si has a lowermelting point than pure aluminum, thereby enabling the direct bondingprocess to be conducted at a lower temperature than the melting point ofaluminum. The two aluminum plates 4 and 5 are pressed into the ceramicpanel 3 with a weight of at least 200 g at a temperature of in excess of700° C. for a period of about one minute in an inert atmosphere ofnitrogen. For further details on the attachment of aluminum plates 4 and5 to ceramic panel 3, and for further details on direct metal bondedsubstrates, see: U.S. Pat. No. 6,798,060, U.S. Pat. No. 7,005,734, andU.S. Pat. No. 6,404,065 (the subject matter of these three patentdocuments is incorporated herein by reference).

FIG. 3 illustrates a subsequent step of patterning and etching thealuminum plates 4 and 5. After patterning using photoresist and astandard photolithographic process, each plate is etched. FeCl₃ basedchemistry or CuCl₂ based chemistry may be used. Etching may leave someresidue of the AlSi alloy. This residue is removed using a mixture ofnitric acid, hydrofluoric acid and acetic acid. The result of thepatterning and etching is a plurality of aluminum islands on the topside of ceramic panel 3, and a plurality of aluminum islands on thebottom side of ceramic panel 3. In the diagram of 3, reference numerals6 and 7 identify two of the islands on the top side of the resultingpanel assembly 9. The corresponding island 8 on the bottom side of panelassembly 9 is not seen in the perspective of FIG. 3.

FIG. 4 illustrates a subsequent step 11 of cleaning the panel assembly 9to remove a native aluminum oxide that develops on the aluminumsurfaces. After the patterning and etching of the aluminum has beencarried out, and panel assembly is generally handled in an airatmosphere. As a consequence of being exposed to air, any exposedaluminum surfaces quickly develop a native aluminum oxide. The cleaningstep of FIG. 4 removes this native aluminum oxide. In the illustratedexample, the panel is immersed in a strong alkaline solution 10 of NaOH,followed by a zinc based treatment.

FIG. 5 illustrates a subsequent step 12 of plating a metal onto thecleaned aluminum surfaces. Once the native aluminum oxide has beenremoved as illustrated in FIG. 4, all exposed aluminum surfaces areimmediately plated with a metal such as nickel or palladium. Anelectroplating process 11 may be used as illustrated in FIG. 5. Theresulting plating metal layers on the aluminum islands are about fivemicrons thick. Alternatively, an electroless nickel plating process maybe used in which case the nickel layer may include about eight percentphosphorous. Alternatively, an electroless palladium plating process maybe used.

FIG. 6 is a simplified cross-sectional diagram of DBA 9 after the metalplating step of FIG. 5 has been performed. Metal layer 13 platesaluminum island 6. Metal layer 14 plates aluminum island 7. Metal layer15 plates aluminum island 8.

FIG. 7 is a table that sets forth the constituents of a silvernanoparticle paste. The listed weight percentages and temperatures areapproximate.

FIG. 8 illustrates a step of depositing volumes of the silvernanoparticle paste 16 of FIG. 7 onto selected regions of themetal-plated aluminum islands on the top side of the panel assembly 9.Alternatively, where the metal layers 13 and 14 are palladium, thesilver nanoparticle paste may be an mAgic Paste Microbond paste, seriesASP016, ASP043, ASP131 or APA859, that is commercially available fromHeraeus Materials Technology GmbH & Co. KG of Hanau, Germany. A screenprinting process using a patterned screen 17 and a wiper 18 may be used.The patterned screen 17 has openings where corresponding volumes ofsilver nanoparticle paste are to be deposited. In another example, thesilver nanoparticle paste may be ink-jetted onto the panel assembly ormay be sprayed.

FIG. 9 is a cross-sectional diagram of one volume 19 of the silvernanoparticle paste after the screen 17 of FIG. 8 has been removed andbefore sintering. The circles in FIG. 9 represent particles of silver.Each silver nanoparticle is coated with the dispersant/binder. Thedispersant/binder and the thinner of the paste serve to keep the silverparticles evenly dispersed in the paste volume. The diagram is not toscale but rather is schematically shown to better illustrate thesintering process.

FIG. 10 is a cross-sectional diagram that illustrates the volume 19midway through the sintering process. As the temperature increases toabout 150° C., the thinner evaporates. This results in a somewhat moredense packing of the nanoparticles as illustrated. Once the thinner hasevaporated, the temperature is increased further to approximately 200°C. The type of dispersant/binder is selected so that thedispersant/binder coating separates from the silver particles and burnsout at this 200° C. elevated temperature, but before actual sinteringtakes place at a higher sintering temperature of 250° C. Generally thedispersant and the binder involve organic molecules involving carbonchains of twelve or more carbon atoms, whereas the thinner is an organicmolecule that has carbon chains of approximately three carbon atoms.Before burning off, these and/or other constituents of the paste maydecompose to form reactive compounds and acids. These reactive compoundsand acids assist in penetrating, cleaning and/or removing oxides fromthe surface upon which the paste is disposed.

After burn out of the organic compounds, the temperature is increased tothe higher sintering temperature of 250° C. The sintering 250° C.temperature causes the silver nanoparticles to densify and to sintertogether. The sintering temperature depends on the size of thenanoparticles and the compaction pressure applied during sintering. Inthe present example, no compaction pressure is applied.

FIG. 11 is a cross-sectional diagram of a resulting sintered silverfeature. There is good electrical contact between the silver of thesintered feature and the unoxidized aluminum of the underlying aluminumisland 6. This electrical connection exists through metal plate layer13. The sintered silver feature is also mechanically bonded to metalplate layer 13. The precise manner of mechanical bonding is not fullyunderstood, but it is believed that some of the small silvernanoparticles in their presintered state fit down into small cracks andimperfections in metal plate layer 13. These embedded particles are thensintered in place so that they are bonded to one another and to othernanoparticles of the larger sintered feature above. Due to thesesintered extensions that are anchored into the cracks in metal platelayer 13, the resulting monolithic sintered feature grasps and is bondedto metal plate layer 13.

FIG. 12 is a perspective view of panel assembly 9 after sintering. Asintered silver feature is disposed on each corresponding metal-platedaluminum island on the top side of the panel. Reference numeral 20identifies the sintered silver feature disposed on aluminum island 6.Reference numeral 21 identifies the sintered silver feature disposed onaluminum island 7. The dashed lines in FIG. 12 indicate where the panelassembly 9 will be cut to form individual DBAs.

FIG. 13 is a cross-sectional diagram of one DBA substrate 22 after theDBA substrate has been cut from the panel assembly 9 of FIG. 12. The DBAsubstrate 22 may, for example, have a width of 14.2 mm and a length of17.0 mm. In this example, there are two metal-plated aluminum islands 6and 7 on the top side of ceramic substrate member portion 40. There isone metal-plated aluminum island 8 on the bottom side of the ceramicsubstrate member portion 40.

Next, a row of ten DBA substrates is welded simultaneously tocorresponding leads of a suitable leadframe. The leads of the leadframe,prior to their being trimmed and separated into individual leads, aremade to extend toward the row of DBA substrates. For each DBA substrate,one corresponding center lead of the leadframe is physically attached tothe top side metal of the DBA substrate. In the present example, thesecenter leads are ultrasonically welded under pressure to the top sideplates of corresponding DBA substrates.

FIG. 14 illustrates a subsequent step of performing directsilver-to-silver die attach bonding. The back side of a single wafer isplated with aluminum, then titanium, then nickel, and finally silver.The silver is deposited onto the back side of the wafer using anevaporative process. The silvered wafer is then cut into individualdice. Semiconductor dice 23 and 24 are two dice cut from this wafer.Reference numerals 25 and 26 identify back side surface silver layersthat are formed by evaporation. A top boat (not shown) made of graphiteis fashioned with recesses to accommodate ten pairs of dice. The dicefit into the recesses in the top boat with their silvered back sidesurfaces facing up. A bottom boat (not shown) made of graphite isfashioned with recesses to accommodate the leadframe and its tenattached DBA substrates. The DBA substrates fit into recesses in thebottom boat with their sintered silver features facing upward. The twoboats are then brought together so that the silvered back sides of thedice register with corresponding silver portions of the DBA substrates.The top boat is pressed downward under a weight of at least one kilogramat a temperature of 250° C. The pressure and temperature causes directsilver-to-silver die attach bonding to occur between the silver backside surfaces of the dice and the sintered silver surfaces of the DBAsubstrates. Die attach is performed in this way on a row of ten DBAsubstrates (that are all attached to one leadframe) at one time due tothe bottom boat accommodating a leadframe with ten attached DBAsubstrates.

FIG. 15 is a simplified perspective view that shows the result of asubsequent wirebonding step. Bond pads 27-30 on the top sides of thedice 23 and 24 are wirebonded via pure aluminum bond wires to variousones of the ends of the leads of the leadframe. Reference numeral 31identifies one such bond wire. In addition, aluminum bond wires areprovided to extend from bond pad 29 to the nickel or palladium platedsurface of island 6. Although only one set of leads 32-36 and one DBAsubstrate 22 is illustrated in FIG. 15, wirebonding is performed on aset of ten DBA substrates that is attached to leads of a singleleadframe. At this point in the manufacturing method, the leads have notyet been trimmed into individual leads but rather are parts of thesingle leadframe. In the diagram of FIG. 15, two flared end extensionsof center lead 34 are shown having been ultrasonically welded to thesintered silver feature 21 on upper surface of plated metal island 7.

FIG. 16 is a circuit diagram of the half-bridge circuit of the assemblyillustrated in FIG. 15. Semiconductor dice 23 and 24 are discreteN-channel power transistor dice. Bond pads 27 and 30 are gate terminalsof the dice. Bond pads 28 and 29 are source terminals of the dice. Theback side silver layer 25 of die 23 is the drain terminal of die 23. Theback side silver layer 26 of die 24 is the drain terminal of die 24. Nosoft solder is used for die attach, nor to attach bond pads to leads.

Next, the set of ten DBA substrates is encapsulated at one time whilethe DBA substrates are still attached to the leadframe. Each DBAsubstrate is encapsulated with a separate amount of injection moldedplastic. Reference numeral 2 identifies an amount of this encapsulantthat encapsulates the DBA substrate 22 of FIG. 15. After encapsulation,lead trimming is performed to cut the leads from one another and fromthe leadframe. The result is ten individual finished DBA-based powerdevices, including packaged power device 1.

FIG. 17 is a perspective view of the finished DBA-based power device 1after encapsulation and lead trimming. There is no soft solder anywhereunder the block of encapsulant 2. Failures that might otherwise occur insevere −55° C. to +150° C. temperature cycling were soft solder bondingused for die attach are avoided due to the solderless, silver-to-silverdie attach employed. Even though the silver nanoparticle paste wassintered at a relatively low temperature of 250° C., silver has a veryhigh melting point of 961° C. The solderless silver-to-silver die attachbond is therefore remains strong even at high temperatures well above200° C.

FIG. 18 is a flowchart of the method 1000 set forth in FIGS. 1-17 ofmaking the DBA-based power device 1 of FIG. 17. Aluminum plates aredirect bonded (step 1001) to a ceramic panel to make analuminum-ceramic-aluminum sandwich structure. The aluminum plates arepatterned and etched (step 1002) to create aluminum islands on the frontside and the back side of the ceramic panel. After any native aluminumoxide is cleaned (step 1003) from the aluminum islands, the aluminumislands are plated (step 1004) with another metal. This other metal maybe a five micron thick layer of nickel of palladium. In one example,there is substantially no native aluminum oxide disposed between thealuminum of the islands and the overlying plated metal. Silvernanoparticle paste is then deposited (step 1005) only onto selectedparts of the metal plated aluminum islands. This step may be generallydescribed as depositing silver nanoparticle paste on to selected partsof the metal plated aluminum islands of a DBA substrate, even throughthe actual DBA substrate at this point in the method is still part ofthe larger DBA panel assembly. Other parts of the DBA panel assemblythat are not to be silvered do not receive silver nanoparticle paste inthis step. The silver nanoparticle paste may be applied by screenprinting or ink-jet printing onto the selected parts of the metal platedaluminum islands. The amount of silver consumed in this silveringprocess is less than were the entire surface of the panel assemblycovered with evaporated silver, and then the silver were patterned andetched to remove much of the deposited silver. In the method of FIG. 18,the only silver deposited is silver that will become a useful part ofthe resulting DBA structures.

Next, the panel assembly is heated to sinter (step 1006) the silvernanoparticle paste, thereby forming sintered silver features on theselected parts of the aluminum islands. After sintering, the panelassembly is cut (step 1007) into a plurality of individual DBAsubstrates. Multiple DBA substrates are then secured (for example, byultrasonic welding) to selected leads of a leadframe (step 1007). Directsilver-to-silver die attach bonding (step 1008) is then performed. Theback side of each die includes a silver layer that was formed byevaporative deposition of silver onto the entire back side of the waferprior to singulation of the wafer into individual dice. The top sides ofthe DBA substrates have corresponding sintered silver features. Thesilvered back side surfaces of the dice are pressed down onto thecorresponding sintered silver features of the DBA substrates. Undersuitable pressure and temperature each die is bonded to a DBA substratewith a solderless silver-to-silver bond. Next, bond pads on the frontsides of the dice are bonded via aluminum bond wires to ends ofcorresponding other leads of the leadframe. In addition, bond wires maybe provided between structures on a DBA substrate. For example, a bondpad on a die disposed on one aluminum island may be wirebonded directlyto the other plated aluminum island of the DBA substrate as illustratedin FIG. 15. After wirebonding, each DBA substrate of the leadframeassembly is then encapsulated (step 1009) in an amount of injectionmolded plastic. Lead trimming is then performed (step 1010) to separatethe leads from one another, thereby forming individual packagedDBA-based power devices.

FIG. 19 is a flowchart of another method 2000 of making a DBA-basedpower device. Aluminum plates are direct bonded to a ceramic panel (step2001) and the aluminum plates are patterned and etched (step 202) as inthe method of FIG. 18. Rather than then plating the aluminum islandswith another metal (for example, nickel or palladium) as described abovein connection with the method of FIG. 18, the aluminum islands are notplated with another metal. Rather, silver nanoparticle paste isdeposited (step 2003) directly onto the aluminum islands. Due to thisprocessing occurring in an air atmosphere, the aluminum islands areactually covered with a thin layer of a native aluminum oxide. Moreprecisely then, the silver nanoparticle paste is deposited onto the thinnative aluminum oxide layers that in turn are on the aluminum islands.After deposition of the silver nanoparticle paste, the individualvolumes of nanoparticle paste are sintered (step 2004) to formindividual sintered silver features on the aluminum islands.

In one example, the native aluminum oxide under a sintered featureremains largely in tact. Current flow into or out of the back side metalof each die passes laterally largely through the sintered silver featurerather than downward through the silver and into the aluminum. Due tothe presence of the aluminum oxide layer, the underlying aluminum is notrelied upon to carry current.

In another example, the native aluminum oxide is penetrated during thesintering process. Better electrical connection is then made betweeneach sintered silver feature and the unoxidized aluminum of theunderlying aluminum island. The native aluminum oxide layer is typicallynot entirely removed, but rather cracks are believed to form in thenative aluminum oxide during the sintering process due to differentialthermal expansion. Pressure and ultrasonic energy can be provided topromote this cracking. In some examples, the pressure and ultrasonicenergy is applied only at the beginning of the sintering process. Thecracks expose amounts of unoxidized aluminum. Bonding between the silvernanoparticles and the exposed aluminum occurs in these cracks.

In another example, the silver nanoparticle paste includes particles ofa flux material. The flux particles are not active at room temperature,but under elevated temperatures of the sintering process the fluxparticles decompose into compounds such as acids. These compounds attackand help penetrate the native aluminum oxide. Pressure and ultrasonicenergy can also be provided to promote cracking of the native aluminumoxide. Bonding between the silver nanoparticles and the exposedaluminum, and/or between the other constituents of the paste and theexposed aluminum, occurs in these cracks. The dispersant/binder and fluxresidue is then burned off or driven off as a consequence of the highersintering temperatures at the end of the sintering process. Aftersintering, the panel assembly is cleaned to remove any flux residue thatmight still remain.

The panel assembly is then cut into DBA substrates (step 2005). The DBAsubstrates are bonded to a leadframe (step 2006), directsilver-to-silver die attach is performed (step 2007), wirebonding isperformed (step 2008), encapsulation is performed (step 2009), andfinally lead trimming is performed (step 2010) resulting in separateindividual DBA-based power devices.

FIG. 20 is a table that sets forth the constituents of the silvernanoparticle paste used in one example of the method of FIG. 19. Thelisted weight percentages and temperatures are approximate.

FIGS. 21-23 illustrate three consecutive steps in the sintering processin which the silver nanoparticle paste of FIG. 20 is employed in themethod of FIG. 19. In the simplified diagram of FIG. 21, the blackcircles represent particles of flux material. Reference numeral 37identifies one such flux particle. The amount 19 of silver nanoparticlepaste is disposed directly on the native aluminum oxide layer 38. Thenative aluminum oxide layer 38 in turn is on aluminum 6 on the top sideof the DBA. FIG. 22 shows the structure of FIG. 21 after the sinteringprocess has begun. The native aluminum oxide 38 is believed to bemechanically activated and crack due to differential thermal expansionsuch as between the aluminum 6 and the aluminum oxide 38. Pressure canbe applied, and an initial amount of ultrasonic vibration used attemperature, to help mechanically disrupt the native aluminum oxide. Theflux particles have decomposed into reactive agents that in turn assistin penetrating the cracks in the native aluminum oxide. Chemicalreactions take place at the bottoms of the cracks between unoxidizedaluminum and constituents of the decomposed flux. At elevatedtemperatures the flux residue and all other organics of the paste areburned out. As the temperature increases further to 250° C., the silvernanoparticles densify and sinter together to form the sintered silverfeature.

FIG. 23 shows the resulting sintered silver feature 20. Native aluminumoxide quickly reforms on any exposed aluminum in the bottoms of thecracks. Accordingly, no cracks are illustrated in the diagram of FIG. 23in the part of the native aluminum oxide that is not covered by feature20. Good electrical contact exists between the sintered silver feature20 and the underlying aluminum 6 of the island. The electricalconnection is believed to exist through holes in the native aluminumoxide layer beneath the sintered silver feature.

FIG. 24 is a flowchart of another method 2000 of making the DBA-basedpower device of FIG. 17. Aluminum plates are direct bonded to a ceramicpanel (step 3001) and the aluminum plates are patterned and etched (step3002) as in the method of FIG. 18. Rather than then plating theresulting aluminum islands with another metal (for example, with nickelor palladium) as described above in connection with the method of FIG.18, the panel assembly is placed in a chamber 41 that is substantiallydevoid of oxygen and moisture. In one example, the chamber has anatmosphere that is substantially entirely nitrogen. Next (step 3003),any native aluminum oxide that has formed on certain selected localizedareas of the aluminum islands is mechanically removed.

FIG. 25 is a cross-sectional diagram showing how this localizedmechanical cleaning is carried out in one example. Two micro-nozzles 42and 43 and a vacuum nozzle 44 are mounted to a micro-jet head. Themicro-jet head is precisely controlled to move in the X, Y and Zdimensions as in known in the micro-jetting arts. Micro-nozzle 42 blastsa stream of abrasive particles with a suitable high velocity at theselected localized area 45 of the aluminum islands as shown. Vacuumnozzle 43 is positioned to remove the spent abrasive particles and otherdebris after the particles have hit the surface of the aluminum island.In this way a high velocity stream of abrasive particles mechanicallyblasts away the native aluminum oxide. The micro-nozzles are moved inthe X, Y and Z dimensions to remove the native aluminum oxide from adesired localized area or areas of the aluminum islands. A layer ofnative aluminum oxide does not immediately regrow on the cleaned areasdue to the entire panel assembly 9 being disposed in the oxygen-freeenvironment in chamber 41.

Next in the method of FIG. 24, silver nanoparticle paste is deposited(step 3004) onto the cleaned aluminum surface or surfaces to cover thecleaned localized area. This deposition also occurs in the oxygen-freeenvironment in chamber 41 such that the panel assembly is not subjectedto oxygen between the time of native oxide removal until after the timeof deposition of the silver nanoparticle paste.

FIG. 26 is a cross-sectional diagram showing how the silver nanoparticlepaste is deposited in one example. Micro-nozzle 44 micro-jets a streamof small microdots of nanoparticle paste onto the cleaned localized areaor areas of the aluminum island or islands. In one example, all suchareas that are later to receive nanoparticle paste are cleaned in step3003, and thereafter the silver nanoparticle paste is applied to allthose areas in step 3004. In another example, one area is cleaned instep 3003 without all such other areas on the panel assembly beingcleaned, and thereafter nanoparticle paste is deposited onto the onecleaned area in step 3004, and then these two steps are repeated for thenext area to be silvered, and so forth. Regardless of the order ofcleaning and applying silver nanoparticle paste to the various areas tobe silvered, the deposited volumes of paste are then sintered (step3005) to form the sintered silver features on the aluminum islands. Eachof the resulting sintered silver features is at least in part disposeddirectly on aluminum without intervening aluminum oxide being disposedbetween the sintered silver and the underlying aluminum of the aluminumisland. The panel is then cut into individual DBA substrates (step3006). DBA substrates are bonded (step 3007) to a leadframe, directsilver-to-silver die attach bonding is performed (step 3008),wirebonding is performed (step 3009), encapsulation is carried out (step3010), and the leadframe is cut (step 3011) thereby forming separateindividual DBA-based power devices.

To reduce the amount of time required to carry out the cleaning andpaste depositing steps, in one example the micro-jet equipment involvesmultiple pairs of abrasive and paste depositing nozzles, where thecleaning nozzle of each pair performs its cleaning task on acorresponding localized area at the same time that the cleaning nozzlesof the other pairs clean other localized areas. Thereafter thedepositing nozzle of each pair performs its depositing task on acorresponding localized area at the same time that the depositingnozzles of the other pairs deposit onto other localized areas. Thevarious pairs of nozzles therefore operate in parallel on differentlocalized areas. A gas pump creates a gas flow through chamber 41. Thisflow of gas carries debris of the abrasion away from the cleanedsurfaces. The abrasive particle material is a dust or a sand involvingalumina powder, silica, or another clean sand blasting powder. If anentire aluminum surface needs to be coated with the silver nanoparticlepaste (for example, the back side of a chip or a substrate), then asimple tool with a small brush or rotating file may be used to strip thenative aluminum oxide, followed immediately by depositing of the silvernanoparticle paste, and followed by firing of the paste to form thedesired sintered silver features. In one example, rather than removing alocalized area of native aluminum oxide using abrasive particles asdescribed above, the cleaning micro-nozzle applies a mildly reactivecleaning liquid (for example, a mildly acidic liquid) to the localizedarea in order to etch away or to penetrate the native oxide. Thisapplication is followed by an inert clean cycle that washes off thecleaning liquid and the residues, thereby readying the cleaned surfacefor nanoparticle paste deposition.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. An assembly, comprising: a Direct-Bonded Aluminum(DBA) substrate, wherein the DBA substrate comprises a ceramic portion,an aluminum layer disposed on the ceramic portion, and a sintered silverlayer disposed over the aluminum portion; and a semiconductor die havinga back side surface and a front side surface, wherein the back sidesurface is a silver surface, and wherein the silver surface of the backside surface of the semiconductor die is silver-to-silver bondeddirectly to the sintered silver layer of the DBA substrate without anyintervening solder, lead, tin, antimony or other metal between thesilver surface and the sintered silver layer.
 2. The assembly of claim1, wherein the assembly is a packaged power device, the assembly furthercomprising: a plurality of leads that are coupled by one or more bondwires to one or more pads on the front side surface of the semiconductordie; and an encapsulant that encapsulates the semiconductor die, thebond wires, part of the leads, and at least part of the DBA substrate,wherein there is no solder encapsulated by the encapsulant.
 3. Theassembly of claim 1, wherein the DBA substrate further comprises a layerof plated metal disposed between the sintered silver layer of the DBAand the aluminum layer of the DBA substrate.
 4. The assembly of claim 3,wherein the layer of plated metal is a layer taken from the groupconsisting of: a nickel layer, and a palladium layer.
 5. The assembly ofclaim 1, wherein there is substantially no native aluminum oxide layerdisposed between the sintered silver layer of the DBA substrate and thealuminum layer of the DBA substrate.
 6. An assembly, comprising: aDirect-Bonded Aluminum (DBA) substrate, wherein the DBA substratecomprises a ceramic portion, an aluminum layer disposed on the ceramicportion, and a sintered silver layer disposed over the aluminum portion;and a semiconductor die having a back side surface and a front sidesurface, wherein the back side surface is a silver surface, wherein thesilver surface of the back side surface of the semiconductor die issilver-to-silver bonded directly to the sintered silver layer of the DBAsubstrate without any intervening solder, and wherein the DBA substratefurther comprises a thin native aluminum oxide layer disposed betweenthe sintered silver layer of the DBA substrate and the aluminum layer ofthe DBA substrate.
 7. The assembly of claim 1, wherein the DBA substratefurther comprises a thin native aluminum oxide layer disposed betweenthe sintered silver layer of the DBA substrate and the aluminum layer ofthe DBA substrate, and wherein the thin native aluminum oxide layer ispenetrated in at least some locations such that a directsilver-to-aluminum connection exists between the sintered silver layerof the DBA substrate and the aluminum layer of the DBA substrate.
 8. Adevice comprising: a Direct-Bonded Aluminum (DBA) substrate, wherein theDBA substrate includes a sintered silver layer disposed over an aluminumlayer; and a semiconductor die having a back side surface and a frontside surface, wherein the back side surface is a silver surface, andwherein there is a solderless silver-to-silver die attach bond betweenthe silver surface of the back side surface of the semiconductor die andthe sintered silver layer of the DBA substrate such that the silversurface is fused directly to the sintered silver layer without anyintervening solder, lead, tin, antimony or other metal between thesilver surface and the sintered silver layer.
 9. The device of claim 8,wherein the device is a packaged power device, the device furthercomprising: a plurality of leads that are coupled by one or more bondwires to one or more pads on the front side surface of the semiconductordie; and an encapsulant that encapsulates the semiconductor die, thebond wires, part of the leads, and at least part of the DBA substrate,wherein there is no solder encapsulated by the encapsulant.
 10. Thedevice of claim 8, wherein the DBA substrate further comprises a layerof plated metal disposed between the sintered silver layer of the DBAand the aluminum layer of the DBA substrate.
 11. The device of claim 10,wherein the layer of plated metal is a layer taken from the groupconsisting of: a nickel layer, and a palladium layer.
 12. The device ofclaim 8, wherein there is substantially no native aluminum oxidedisposed between the sintered silver layer of the DBA substrate and thealuminum layer of the DBA substrate.
 13. A device comprising: aDirect-Bonded Aluminum (DBA) substrate, wherein the DBA substrateincludes a sintered silver layer disposed over an aluminum layer; and asemiconductor die having a back side surface and a front side surface,wherein the back side surface is a silver surface, wherein there is asolderless silver-to-silver die attach bond between the silver surfaceof the back side surface of the semiconductor die and the sinteredsilver layer of the DBA substrate such that the silver surface is fuseddirectly to the sintered silver layer, and wherein the DBA substratefurther comprises a thin native aluminum oxide layer disposed betweenthe sintered silver layer of the DBA substrate and the aluminum layer ofthe DBA substrate.
 14. The device of claim 13, wherein the sinteredsilver layer is formed by sintering an amount of silver nanoparticlepaste over the thin native aluminum oxide layer such that a directsilver-to-aluminum connection is formed between the sintered silverlayer and at least some parts of the aluminum of the aluminum layer. 15.The device of claim 8, wherein the DBA substrate further comprises athin native aluminum oxide layer disposed between the sintered silverlayer and the aluminum layer, and wherein the thin native aluminum oxidelayer is penetrated in at least some locations such that a directsilver-to-aluminum connection exists between the sintered silver layerand the aluminum layer.
 16. The device of claim 8, further comprising:an encapsulant that encapsulates the semiconductor die and at least partof the DBA substrate, wherein there is no solder encapsulated by theencapsulant.